Thermoplastic materials, such as hot melt adhesive, are dispensed and used in a variety of applications including the manufacture of diapers, sanitary napkins, surgical drapes, and various other nonwoven products. This technology has evolved from the application of linear beads or fibers of material and other spray patterns, to air-assisted applications, such as spiral and melt-blown depositions of fibrous material.
Known adhesive applicators used for dispensing such thermoplastic materials may include one or more valve modules for applying the intended deposition pattern of adhesive, each valve module having valve components that operate in an on/off fashion. One example of a valve module is disclosed in U.S. Pat. No. 6,089,413, assigned to the assignee of the present invention, and the disclosure of which is hereby fully incorporated by reference herein in its entirety. This module includes valve structure which switches the module between ON and OFF conditions relative to the dispensed material.
In the ON condition, the module is in a dispensing mode in which pressurized liquid material fed into the module through a liquid inlet passage is directed through a dispensing outlet passage and into a dispensing nozzle for deposition onto the substrate. In the OFF condition, the module switches into a recirculating mode in which the pressurized liquid material fed into the module is redirected to a recirculation outlet passage and into a recirculation channel in a manifold of the applicator. The liquid material is transferred through the recirculation channel of the manifold and then through a recirculation conduit leading back toward an adhesive supply reservoir located remotely from the applicator. Recirculating undispensed liquid material during the OFF condition advantageously prevents excessive pressure buildup within the module, which would otherwise distort the shape of the next pattern of liquid material dispensed when the module returns to the ON condition.
During the ON condition, the liquid material flowing through the module is exposed to a first pressure, referred to herein as a “dispensing pressure” (also known as an “application pressure”), as it is forced through the dispensing outlet passage and the dispensing nozzle. The dispensing pressure is a combined result of a flow rate pressure and a dispensing backpressure. The flow rate pressure is a function of forces exerted on the supply material by a liquid pump operating at a given liquid flow rate. The dispensing backpressure is a function of forces exerted on the liquid material by the inner surfaces of the passages and chambers through which the liquid material is forced during dispense, including the dispensing outlet passage and the internal passages of the dispensing nozzle.
During the OFF condition, the liquid material flowing through the module is exposed to a second pressure, referred to as a “recirculation pressure,” as it is redirected through the recirculation outlet passage and into the recirculation channel of the manifold. The recirculation pressure is a combined result of the flow rate pressure and a recirculation backpressure. As described above, the flow rate pressure is a function of the liquid flow rate at which the liquid pump is operating. The recirculation backpressure is a function of forces exerted on the liquid material by the inner surfaces of the passages and chambers through which the liquid material is forced during recirculation, including the recirculation outlet passage and the recirculation channel.
In known valve modules, the dispensing backpressure experienced by the liquid material during the ON condition is generally greater than the recirculation backpressure experienced during the OFF condition. Due to the amount time required for the module to shift its valve components between the OFF (recirculating) and ON (dispensing) conditions, the differential between the dispensing pressure and recirculation pressure acts to hinder the ability of the module to dispense with accurate volumetric outputs at the start of a dispense cycle in the ON condition.
Known dispensing systems include an applicator having a manifold fitted with one or more valve modules along a length of the applicator. On example of such an applicator is disclosed in U.S. Pat. No. 6,422,428, assigned to the assignee of the present invention, and the disclosure of which is hereby fully incorporated by reference herein in its entirety. Such dispensing systems allow the flexibility for one or more of the valve modules on the applicator to be operated at a unique liquid flow rate and/or to be fitted with a dispensing nozzle that yields a unique dispensing backpressure during use. Accordingly, one or more of the modules on the applicator may operate with a unique pressure differential caused by a unique dispensing pressure and/or a unique recirculation pressure.
Known dispensing systems may also include a single backpressure control valve, positioned remotely from the applicator near the liquid supply reservoir, and operable to control a backpressure within the recirculation conduit with which each of the modules communicates. However, this single control valve is incapable of controlling a backpressure within each module individually, and thus is ineffective to neutralize unique pressure differentials across multiple modules on the applicator. As such, a significant pressure differential remains in one or more of the valve modules, which negatively affects dispensing performance for that module(s), as described above.
Accordingly, a need remains for improvement in liquid dispensing applicators to address the present challenges and shortcomings such as those described above.